Unstable Particles as Targets in Scattering Experiments

A general method is suggested for analyzing the scattering of particle $A$ by particle $B$, leading to three or more final particles, in order to obtain the cross section for the interaction of $A$ with a particle which is virtually contained in $B$. Binding complications are absent if a plausible assumption about the location and residues of poles in the $S$ matrix is accepted. The method is useful for unstable particles from which free targets cannot be made; the special examples of pion and neutron targets are discussed in detail.